A ball screw with balls capable of rolling between a screw shaft and a nut enables a reduction in coefficient of friction in rotating the screw shaft with respect to the nut as compared with a screw coming into sliding contact, and therefore, is in practical use in a positioning mechanism of a machine tool, feeding mechanism, steering gear of an automobile and the like.
In recent years, to increase an allowable load, a roller screw has been devised which uses rollers as rolling members, instead of balls, as in Patent Document 1. The roller screw uses a return pipe to circulate the rollers. Each of the rollers rolling in a roller rolling groove of the screw shaft is scooped by the return pipe, passed through the return pipe, returns to an original position, and circulates infinitely. The return pipe is formed in the shape of a substantially gate in the entire shape, and formed of a center portion and the opposite end portions bent toward the center portion. Bonding portions between the center portion and opposite end portions are formed in the shape of an arc to circulate the rollers smoothly.    Patent Document 1: Japanese Laid-Open Paten Publication No. 2001-241527